The present invention relates to a semiconductor imaging device including micro lenses at the surface thereof, and particularly relates to a semiconductor imaging device that suppresses image failure caused due to field curvature and a method for manufacturing it.
Recently, as demand for high-density packaging of semiconductor components increases in association with reduction in size and thickness of electronic appliances, novel structures and manufacturing methods are being examined for attaining a high-quality, small-size, and thin semiconductor imaging devices in which semiconductor imaging elements are incorporated.
In general, a semiconductor imaging device is so structured that micro lenses of convex lenses are provided on a light receiving face for increasing sensitivity of the semiconductor imaging device to focus incident light on the light receiving face for effective light collection. In the semiconductor imaging device, an optical lens system is arranged in front of the imaging face. The arrangement of the optical lens system in front of the imaging face, however, involves blurring of an image and color bleeding, which are caused due to field curvature. Though the shape of the lenses and the diaphragm of the optical lens system are optimized for preventing blurring and bleeding, such countermeasures on the optical lens system itself make the shape of the lenses and the structure as a whole to be complicated and invite an increase in cost. For this reason, structures and methods which achieve compensation of the field curvature of the optical lens system at lower cost are desired.
In response to the above desire, a method has been proposed as a first example in which a semiconductor imaging element having a thickness of 20 μm or smaller in a bear chip state is allowed to adhere and be fixed to a mounting concave portion of a wiring substrate (see Patent Document 1: Japanese Patent Application Laid Open Publication No. 2003-244558A). The mounting concave portion is formed in, for example, an arc shape or a spherical curved shape at a predetermined curvature, and the semiconductor imaging element is fitted along the mounting concave portion to be in a shape curved at a predetermined curvature. For obtaining such a thin film semiconductor imaging device, the document also discloses a method in which after forming a solid-state imaging element on an epitaxial layer grown on a porous silicon layer provided on a silicon substrate, the silicon substrate is separated at the porous silicon layer.
With the above structure, a semiconductor imaging device including a curved semiconductor imaging element can be realized easily at good yield, and an electronic appliance in which it is incorporated can be reduced in size.
Referring to a second example, there has been proposed micro lenses that can compensate the field curvature of an optical lens system and a method of manufacturing them (see, for example, Patent Document 2: Japanese Patent Application Laid Open Publication No. 9-260624A). In this method, a lens formation layer is etched with the use of resist patterns having convex portions each for each pixel as masks. Therefore, appropriate selection of the etching method, for example, etching at an etching rate to the resist pattern substantially equal to that to the lens formation layer forms the lens formation layer having the same convex portions as the resist patterns. Further, the resist patterns are patterned so as to have analog patterns and so that the areas of patterns arranged in the peripheral part of the imaging face become larger than those arranged in the central part of the imaging face. Accordingly, when the lens formation layer is etched with the use of the resist patterns having such the patterns as masks, the micro lenses are formed so that the curvatures of the convex lenses arranged in the peripheral part of the imaging face are larger than the curvatures of the convex lenses arranged in the central part thereof.
With the micro lenses having the structures in which the convex lenses arranged in the peripheral part of the imaging face have curvatures larger than the convex lenses arranged in the central part thereof, when the curvatures of the convex lenses are made correspondence with the optical lens system arranged in front of the semiconductor imaging element, aberration of the field curvature of the optical lens system can be compensated by the micro lenses.
In the semiconductor imaging device of the first example, however, the curved shape is obtained by bending the semiconductor imaging element by pushing the upper surface of the semiconductor imaging element physically by means of a pressing jig including an elastic pad in packaging the semiconductor imaging element. This method means that the surfaces of the micro lenses made of transparent resin are pressed by the pressing jig at heating for fixing the semiconductor imaging element by an adhesive onto a wiring substrate, inviting deformation of the surfaces of the micro lenses, adhesion of minute dust, and the like. This requires the pressing step to be performed in clean environment. Further, the semiconductor imaging element processed to be thin, approximately 20 μm must be handled and presses against the mounting concave portion so as not to generate a crack under adequate management, which leads to poor productivity.
Referring to the second example, which is a method in which the curvatures of the micro lenses are changed, thinning the semiconductor imaging element and strict process management in the adhesion step as in the first example are unnecessary. Formation of the resist patterns having the patterns of the convex portions for the respective pixels, however, requires melting of the resist patterns so that the central part of each pixel is upwardly convex by its surface tension. Further, etching must be performed under the condition where the etching rate to the resist patterns is substantially equal to that to the lens formation layer. In order to satisfy these requirements, the materials of the resists and the lens formation layer are limited. Moreover, patterning must be performed so as to set the areas of the resist patterns arranged in the peripheral part of the imaging face on the light receiving section are larger than those of the resist patterns arranged in the central part of the imaging face thereon. The difference in shape of the resist patterns leads to complicated design for shape and the like in the central part of the light receiving section and in the peripheral part thereof.